1. Field of the Invention
The invention relates to novel crosslinkable, photoactive silane derivatives with 3-arylacrylic esters and 3-arylacrylamides, and their use as orientation layers for liquid crystals and for the production of unstructured or structured optical elements and multilayer systems.
The orientation layer is particularly important in (electro-optical) liquid crystal devices. It serves for ensuring uniform and trouble-free orientation of the longitudinal axes of the molecules.
2. Description of the Prior Art
Uniaxially rubbed polymer orientation layers, such as, for example, polyimide, are usually used for orienting liquid crystal molecules in liquid crystal displays (LCDs). The direction of rubbing determines the orientation direction in this process. However, rubbing entails some serious disadvantages which may strongly influence the optical quality of liquid crystal displays. Thus, rubbing produces dust which may lead to optical defects in the display. At the same time, the polymer layer is electrostatically charged, which, for example in thin film transistor (TFT)-TN-LCDs, may result in the destruction of the thin film transistors underneath. For these reasons, the yield of optically satisfactory displays in LCD production has not been optimal to date.
A further disadvantage of rubbing is that it is not possible to produce structured orientation layers in a simple manner since the orientation direction cannot be varied locally during rubbing. Thus, mainly layers uniformly aligned over a large area can be produced by rubbing. However, structured orientation layers are of considerable interest in many areas of display technology and integrated optics. For example, the dependency of the angle of view of twisted nematic (TN) LCDs can thus be improved.
Orientation layers in which the orientation direction can be predetermined by exposure to polarized light have been known for some time. The problems inherent in rubbing can thus be overcome. In addition, it is possible to specify the orientation direction differently from region to region and hence to structure the orientation layer.
One possibility for the structured orientation of liquid crystals utilizes the isomerizability of certain dye molecules for inducing a preferred direction photochemically by exposure to polarized light of suitable wavelength. This is achieved, for example, by mixing a dye with an orientation polymer and then exposing said dye to polarized light. Such a guest/host system is described, for example, in U.S. Pat. No. 4,974,941. In this system, azobenzenes are mixed into polyimide orientation layers and then exposed to polarized light. Liquid crystals which are in contact with the surface of a layer exposed in this manner are oriented according to this preferred direction. This orientation process is reversible, i.e. the already established direction of orientation can be rotated again by further exposure of the layer to light having a second polarization direction. Since this reorientation process can be repeated as often as desired, orientation layers of this type are less suitable for use in LCDs.
A further possibility for producing highly resolved orientation patterns in liquid crystalline layers is described in Jpn. J. Appl. Phys. Vol. 31 (1992), 2155. In this process, the dimerization of polymer-bound photoreactive cinnamic acid groups, induced by exposure to linearly polarized light, is utilized for the structured orientation of liquid crystals. In contrast to the reversible orientation process described above, an anisotropic polymer network is established in the case of the photostructurable orientation layers described in Jpn. J. Appl. Phys. Vol. 31 (1992), 2155. These photo-oriented polymer networks can be used wherever structured or unstructured liquid crystal orientation layers are required. Apart from in LCDs, such orientation layers can also be used, for example, for the production of so-called hybrid layers, as exemplified in European Patent Applications EP-A-0 611 981, EP-A-0 689 084 and EP-A-0 689 065 and in Swiss Patent Application No. 2036/95. With these hybrid layers of photostructured orientation polymers and crosslinkable low molecular weight liquid crystals, it is possible to realize optical elements, such as, for example, nonabsorptive color filters, linear and circular polarizers, optical retardation layers, etc.
EP-A-611,786 describes cinnamic acid polymers which are suitable in principle for the production of such anisotropically crosslinked, photostructured orientation layers for liquid crystals. These crosslinkable cinnamic acid derivatives are in principle linked to the polymer main chain via the carboxyl function of the cinnamic acid (phenylacrylic acid) and a spacer. However, the photopolymers of this type which have been used to date have a number of serious disadvantages. Thus, for example, photochemical competing reactions adversely affect the orientability. In addition, the known cinnamic acid polymers have insufficient photochemical long-term stability. For example, prolonged exposure of a prefabricated orientation layer to UV light leads to the destruction of the orientation originally present. Multiple exposures in which an existing orientation layer having a predetermined recorded pattern is exposed again in order to orient the still unexposed parts in another direction can be carried out only if the previously exposed parts are covered by a mask. Otherwise, the already oriented parts of the layer may lose some or all of their structure as a result of photochemical secondary reactions.
A further disadvantage of the cinnamic acid polymers used to date is that there is no tilt angle in the case of the orientation surfaces comprising these materials, which surfaces are produced by simple exposure to polarized light. Particularly for use in LCDs, however, a tilt angle must also be provided by the orientation layer in addition to the orientation direction.
In the above-mentioned uniaxially rubbed polymer orientation layers, this tilt angle is already generated in the rubbing process on the polymer surface. If a liquid crystal is brought into contact with such a surface, the liquid crystal molecules are not parallel but inclined to the surface and the tilt angle is thus transmitted to the liquid crystal. The magnitude of the tilt angle is determined both by rubbing parameters, such as, for example, feed rate and pressure, and by the chemical structure of the polymer. For the production of liquid crystal displays, tilt angles between 1.degree. and 15.degree. are required, depending on the type. The larger tilt angles are required in particular for supertwisted nematic (STN) LCDs, in order to avoid the formation of so-called fingerprint textures. In TN and TFT-TN-LCDs, the direction of rotation and the tilting direction are defined by the tilt angle, with the result that "reverse twist" and "reverse tilt" phenomena are prevented. While reverse twist in the unswitched state results in regions with an incorrect direction of rotation, which is manifested visually in a mottled appearance of the display, reverse tilt is optically very troublesome, especially on switching the LCD by tilting the liquid crystals in different directions. Reverse twist can be prevented by doping the liquid crystal mixture with a chiral dopant of suitable direction of rotation. For suppressing reverse tilt, however, there is to date no alternative possibility to the use of orientation layers with a tilt angle.
Recently, cinnamic esters which are not bonded, as described above, to a polymer skeleton but are linked by the spacers to a trialkoxysilane group were reported in Liq. Cryst. 20, 171 (1996). Here, the trialkoxysilane group serves for anchoring the cinnamic acid unit to the substrate as a carrier, for example of glass. The spacer which links the trialkoxysilane group to the cinnamic ester is always attached in the 2-position (ortho position) of the cinnamic ester. For the production of the orientation layer, the trialkoxysilanes are first applied to the glass carrier from a solution. The orientation is then effected by exposure to linearly polarized light of 259 nm wavelength. The ability of the layer prepared in this manner to orient liquid crystal is ascribed to reversible Z/E isomerization. If, on the other hand, the cinnamic acid molecules are exposed at 330 nm, they are crosslinked. Orientability is lost in proportion to the degree of crosslinking.
The orientation layers produced in this manner have the same disadvantages as the cinnamic acid polymers described further above. They too have insufficient photochemical and thermal stability since the Z/E isomerization is reversible and therefore lead to problems associated with reorientation on multiple exposure. In addition, they too lack the ability to induce tilt angles.